NMI to let the active process save its state. Then the busses were taken over during instruction fetch. Feeding NOPs when nothing to do and appropriate instructions when needed. No jumps nor branches. Just the essential processing. A normal interrupt routine couldn't have a tight loop fast enough, so control flow was handled in parallel externally in programmable logic.

Very cool, I highly doubt anybody will ever fully appreciate what a neat hack that is. So you basically divorced the busses from the CPU for a while making it run in a little parallel universe of it's own.

A total perspective vortex of a sort ;)

I realize it would take some more to clearly explain the intricacies, but I see you got it :-)